All questions of The Mitotic Cell Cycle for Grade 11 Exam

Most organisms that undergo sexual reproduction contain two types of cells in their body – haploid and diploid cells. Diploid cells are seen in most parts of the body and contain two copies of every chromosome. Therefore, they contain two genes to determine the sequence of nearly every RNA or protein. Haploid cells are usually male or female gametes formed as a result of meiosis and are seen in sexual organs. They contain only one complete set of chromosomes and are designed to fuse with another haploid gamete to produce a diploid zygote – with one paternal and one maternal set of chromosomes. Though they code for the same genes, their DNA sequences can vary slightly from one another. For instance, the paternal chromosome may contain the genetic information to result in antigen A being present on red blood cells, while the maternal chromosome may code for antigen B. These two antigens are similar in function, but differ from each other marginally in their amino acid sequence.

Mitosis is called equational division because during mitosis the cell divides equally into two identical daughter cells. 

Mitosis or the equational division is usually restricted to the diploid cells only. However, in some lower plants and in some social insects haploid cells also divide by mitosis. It is very essential to understand the significance of this division in the life of an organism. Are you aware of some examples where you have studied about haploid and diploid insects? Mitosis results in the production of diploid daughter cells with identical genetic complement usually. The growth of multicellular organisms is due to mitosis. Cell growth results in disturbing the ratio between the nucleus and the cytoplasm. It therefore becomes essential for the cell to divide to restore the nucleo-cytoplasmic ratio. A very significant contribution of mitosis is cell repair. The cells of the upper layer of the epidermis, cells of the lining of the gut, and blood cells are being constantly replaced. Mitotic divisions in the meristematic tissues – the apical and the lateral cambium, result in a continuous growth of plants throughout their life.

The crossing over takes place during the pachytene stage of prophase I of meiosis.

Explanation:
Prophase I is the first stage of meiosis I, which is further divided into five substages: leptotene, zygotene, pachytene, diplotene, and diakinesis. During the prophase I, homologous chromosomes pair up and exchange genetic material through a process known as crossing over. This results in the formation of recombinant chromosomes that carry a combination of genetic material from both parents.

The crossing over occurs during the pachytene stage, which is characterized by the following events:

1. Homologous chromosomes pair up: The homologous chromosomes come together and form a bivalent or a tetrad.

2. Synapsis: The paired homologous chromosomes undergo synapsis, which is the physical association of the homologs.

3. Crossing over: The chromatids of the homologous chromosomes exchange genetic material at specific sites known as chiasmata. This leads to the formation of recombinant chromosomes.

4. Formation of the synaptonemal complex: The synaptonemal complex holds the homologous chromosomes together and facilitates the exchange of genetic material.

5. Nuclear envelope breakdown: The nuclear envelope breaks down, and the spindle fibers attach to the chromosomes.

In conclusion, the crossing over takes place during the pachytene stage of prophase I of meiosis.

The correct answer is option 'A', the cell plate.

Plant cells have a unique feature called the cell wall, which provides support and protection to the cell. During cell division, a new cell wall is formed to separate the two daughter cells. This process is known as cytokinesis.

- Cell Division in Plant Cells:

1. Prophase: In the first stage of cell division, the nuclear membrane starts to disintegrate, and the chromosomes become visible. Each chromosome consists of two identical copies called chromatids, which are held together by a structure called the centromere.

2. Metaphase: The chromosomes align at the center of the cell, forming a metaphase plate. This alignment ensures that each daughter cell receives an equal number of chromosomes during division.

3. Anaphase: The chromatids separate and move towards opposite poles of the cell. This movement is facilitated by the spindle fibers, which attach to the centromere of each chromatid.

4. Telophase: As the chromatids reach the opposite poles, the nuclear membranes start to reform around them. At this stage, the cell begins to prepare for cytokinesis.

- Cytokinesis in Plant Cells:

In animal cells, cytokinesis occurs through a process called cleavage furrow formation, where a contractile ring pinches the cell membrane inwardly. However, plant cells have a rigid cell wall that cannot be constricted in the same way.

1. Cell Plate Formation: Instead of a cleavage furrow, plant cells form a structure called the cell plate during cytokinesis. The cell plate is made up of vesicles containing cell wall materials, such as cellulose and other polysaccharides.

2. Golgi Vesicles: Golgi vesicles containing cell wall materials accumulate at the center of the cell, between the two daughter nuclei.

3. Fusion of Vesicles: The Golgi vesicles fuse together, forming a continuous structure known as the cell plate. This structure gradually expands towards the periphery of the cell, eventually reaching the cell membrane.

4. Cell Wall Deposition: Enzymes are secreted into the cell plate, which catalyze the deposition of cellulose and other components of the cell wall. As the cell plate expands and fuses with the existing cell wall, it forms a new cell wall between the two daughter cells, separating them completely.

In conclusion, the new cell wall in plant cells begins to form through the process of cytokinesis, specifically through the formation and expansion of the cell plate. The Golgi vesicles fuse together to create the cell plate, which then undergoes cell wall deposition to form a new cell wall between the daughter cells.

Plant cells divide by a cell plate that eventually becomes the cell wall whereas animal cells divide by a cleavage furrow.

The nuclear membrane, also called the nuclear envelope, is a double membrane layer that separates the contents of the nucleus from the rest of the cell. It is found in both animal and plant cells. A cell has many jobs, such as building proteins, converting molecules into energy, and removing waste products. The nuclear envelope protects the cell’s genetic material from the chemical reactions that take place outside the nucleus. It also contains many proteins that are used in organizing DNA and regulating genes.

1)Microfilaments collect in the middle region of the cell below the cell membrane. The furrow deepens centripetally and cleaves the cell into two daughter cells. This method is called cleavage cytokinesis. 2)Cytokinesis in plants occurs by cell plate formation. 3)The phragmoplast is a plant cell specific structure that forms during late cytokinesis. Hope it helps!!!!!

Crossing over occurs between prophase 1 and metaphase 1 and is the process where homologous chromosomes pair up with each other and exchange different segments of their genetic material to form recombinant chromosomes. It can also happen during mitotic division, which may result in loss of heterozygosity.

Explanation:
Mitosis is a process of cell division that occurs in somatic cells, resulting in the formation of two identical daughter cells. It consists of several distinct phases, which occur in a specific order.

The correct order of phases in mitosis is Prophase, Metaphase, Anaphase, and Telophase. This order is represented by option B.

Prophase:
- The first phase of mitosis.
- Chromatin condenses into visible chromosomes.
- Nuclear envelope disintegrates.
- Spindle fibers begin to form.

Metaphase:
- The second phase of mitosis.
- Chromosomes align along the equatorial plane of the cell, known as the metaphase plate.
- Spindle fibers attach to the centromere of each chromosome.

Anaphase:
- The third phase of mitosis.
- Sister chromatids separate and move towards opposite poles of the cell.
- Spindle fibers shorten, pulling the chromatids apart.

Telophase:
- The fourth and final phase of mitosis.
- Chromosomes reach their respective poles.
- Nuclear envelope reforms around each set of chromosomes.
- Chromosomes begin to decondense.
- Cytokinesis, the division of the cytoplasm, also occurs during telophase, resulting in the formation of two daughter cells.

Therefore, the correct order of phases in mitosis is Prophase, Metaphase, Anaphase, and Telophase (option B).

And Reason are true and Reason is the correct explanation of Assertion.
2. If both Assertion and Reason are true but Reason is not the correct explanation of Assertion.
3. If Assertion is true but Reason is false.
4. If Assertion is false but Reason is true.

The correct option is 3. If Assertion is true but Reason is false.

The synaptonemal complex does develop between two synapsed homologous chromosomes during meiosis, but it is not required for mitosis. Mitosis is the process of cell division that occurs in somatic cells, where a cell divides into two identical daughter cells. Synapsis and the formation of a synaptonemal complex only occur during meiosis, specifically during prophase I, to facilitate the pairing and exchange of genetic material between homologous chromosomes. Therefore, the Reason is incorrect.

Endomitosis involves multiplication of chromosomes without karyokinesis and cytokinesis.

Understanding the Cell Cycle
The cell cycle is a series of stages that a cell goes through leading to its division. Let's analyze the statements provided:
i. DNA replication occurs continuously throughout the cell cycle.
- This statement is incorrect. DNA replication specifically occurs during the S phase (Synthesis phase) of interphase, not continuously throughout the entire cell cycle.
ii. The cell cycle consists of a series of stages that lead to the division of a cell into two daughter cells.
- This statement is correct. The cell cycle includes stages such as G1, S, G2, and M phase, which culminate in cell division (mitosis).
iii. Cell growth in terms of cytoplasmic increase is a continuous process.
- This statement is also correct. Cell growth does occur continuously as cells prepare for division, with gradual increases in cytoplasmic volume.
iv. The distribution of replicated chromosomes to daughter nuclei is a simple and straightforward event.
- This statement is misleading. The distribution of chromosomes during mitosis is a complex process involving several checkpoints and mechanisms to ensure accuracy.
Conclusion
Based on the evaluations:
- Correct Statements: ii and iii
- Incorrect Statements: i and iv
Thus, the correct answer is option a) ii and iii. The cell cycle is indeed a structured series of stages leading to division, with continuous growth occurring, while DNA replication is not continuous and chromosome distribution is complex.

Anaphase-I involves the splitting of homologous chromosomes.

Complete disintegration of the nuclear envelope marks the beginning of metaphase chromosomes at this phase have two distinct chromatids.

Overview of Cell Cycle Phases
The cell cycle consists of several phases, including G1, S, G2, and M. Each phase has distinct roles in cell growth and division.
Statement I: G1 Phase
- The G1 phase is critical for cell growth.
- During this phase, the cell is metabolically active, producing RNA and synthesizing proteins.
- Importantly, DNA replication does not occur in G1; instead, it prepares the cell for the S phase, where DNA synthesis takes place.
Statement II: G2 Phase
- The G2 phase follows DNA synthesis and focuses on preparing the cell for mitosis.
- During G2, protein synthesis continues, particularly those proteins necessary for cell division, including components of the mitotic spindle.
- Cell growth also continues in this phase, ensuring that the cell is ready for the M phase (mitosis) where actual division occurs.
Conclusion
Both statements accurately describe their respective phases:
- G1 Phase: Metabolic activity and growth occur without DNA replication.
- G2 Phase: Preparations for mitosis take place with ongoing protein synthesis and further cell growth.
Thus, the correct answer is option 'C': Both I and II are correct. Understanding these phases is crucial for grasping cellular processes, especially in the context of the NEET syllabus.

In late prophase, the nuclear envelope, nucleolus, Golgi complex, and endoplasmic reticulum disappear, allowing the chromosomes to become fully condensed and visible. This marks the transition into metaphase, where the mitotic spindle fully forms.

Each chromosome of bivalent attachments with a single spindle in Metaphase I. In Metaphase I one chromosome from each homologous pair migrate towards the pole.

Dictyotene occurs in oocyctes.